Effect of dam construction on nutrient deposition from a small agricultural karst catchment

Abstract

Abstract Assessing the effects of dam construction on nutrient deposition in small catchments is important to our understanding of the influence of dam interception on downstream nutrient transport as well as of the link between nutrient loss from sloping land and the eutrophic status of large rivers. This study investigates the effect of dam construction on nutrient deposition from a small agricultural karst catchment over the past 50 years by examining the nutrients preserved in reservoir sediments. A new method is proposed to determine the historical nutrient deposition rate ( NDR ) in a small man-made reservoir. We analyzed the concentrations of total nitrogen (TN), total phosphorus (TP), and total organic carbon (TOC) in three sediment cores retrieved from the reservoir, and estimated the mean annual NDRs across the whole reservoir for five depth ranges represented deposition over the periods 1959–1962, 1963–1972, 1973–1985, 1986–1997, and 1998–2012. The relatively low TN, TP, and TOC concentrations below about 30 cm in the three cores reflect the small contribution from anthropogenic nutrient inputs prior to the 1990s. In contrast, the rapid up-core increases in TN, TP, and TOC above 30 cm reflect the increasing supply of anthropogenic nutrients since the 1990s. Natural nutrients were the main contributor to nutrient deposition in the reservoir over the past 50 years. The lack of coupling between sedimentation rates ( SRs ) and nutrient concentrations suggests that the sediment yield has had little influence on the changing pattern of nutrient concentrations preserved within the reservoir deposits, and this provides a new insight into the relationship between sediment yield and nutrient concentration in reservoir sediments. The deposition rates of TN, TP, and TOC over the past 50 years show similar patterns; i.e., they decreased between 1959 and 1972, and then generally increased thereafter. The temporal patterns of these deposition rates were controlled mainly by the sediment yield over the first four periods, whereas anthropogenic nutrient sources (e.g., fertilizer application) were the main cause of the increased NDRs during the final period. These results provide an improved understanding of the environmental factors controlling changes in NDRs . Overall, the profiles of TN, TP, and TOC concentrations, and the NDRs estimated from the three cores, reflect the processes of nutrient loss from the catchment and enrichment in the reservoir in light of environmental change influenced by human activity, and indicate the effect of dam construction on nutrient deposition in the catchment over the past 50 years. This is the first study to document historical NDRs in a small dam reservoir, and the newly proposed method could be applied to other reservoirs in karst and non-karst areas.